© 2014. Published by The Company of Biologists Ltd | Development (2014) 141, 3298-3302 doi:10.1242/dev.111484
MEETING REVIEW
Coordinating cell polarity: heading in the right direction?
Jeffrey D. Axelrod1, * and Dominique C. Bergmann2
A diverse group of researchers working on both plant and animal
systems met at a Company of Biologists workshop to discuss
‘Coordinating Cell Polarity’. The meeting included considerable free
discussion as well as presentations exploring the ways that groups of
cells in these various systems achieve coordinated cell polarity. Here,
we discuss commonalities, differences and themes that emerged
from these sessions that will serve to inform ongoing studies.
KEY WORDS: Cell polarity, Auxin, Eukaryotes
Introduction
In May 2014, The Company of Biologists hosted a workshop
entitled ‘Coordinating Cell Polarity’, which took place at Wiston
House, a grand old estate in Sussex, England. Within this idyllic
setting, the eclectic nature of the group, consisting of about 30
participants working across a broad range of disciplines (including
experimentalists and modelers, working on both plant and animal
systems), was intended to promote the goal of exploring broad
themes in collective cell polarity in both plants and animals.
Reflecting one of the emerging themes – the spontaneous
emergence of polarity – the meeting itself was self-organizing,
with the first session devoted to defining major questions in the
field, from which the speaking order emerged.
Here, we review some themes of discussion, beginning with the
notion of polarization within individual cells, and continuing
toward increasing levels of complexity. We will attempt to point out
some commonality of principles of coordinated cell polarization
despite implementation with distinct molecular machineries, as well
as to discuss some of the big unanswered questions.
A diversity of biological examples of polarity was represented by
participants from both experimental and theoretical or modeling
disciplines. Among the first order of business was to define for the
group how to discuss polarity conceptually, what the premier
models to study polarity are, and what the different models can teach
us. Despite considerable discussion of the distinction between
anisotropy and polarity, no consensus emerged. For example, some
argued that anisotropy (deviation from purely symmetric or
isotropic form or growth) was equivalent to what we commonly
call polarity, whereas others contended that anisotropy was merely a
prerequisite for polarity and that directionality was required for true
polarity. In other words, the debate concerned whether a cell could
have a ‘front’ without also defining a separate ‘back’. Similarly, the
group discussed whether anisotropy is a polarity if no function is
attributed to it, or if it only becomes cell polarity when we are aware
of some importance to the anisotropy. Finally, computational
biologist Przemyslaw Prusinkiewicz (University of Calgary,
Canada) pointed out that, in spite of the wide use of the term,
1
Department of Pathology, Stanford University School of Medicine, Stanford,
2
CA 94305, USA. Department of Biology and HHMI, Stanford University, Stanford,
CA 94305, USA.
*Author for correspondence ( [email protected])
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there is no satisfactory mathematical definition of polarity. This
hinders, in particular, the efforts to model polar phenomena at the
scale of entire tissues.
Common polarity models
Among the diverse audience, two central experimental systems were
most highly represented in the discussions: the localization and
function of the hormone auxin in plant polarity, and the collection of
polarly localized transmembrane proteins that interact to constitute
the Drosophila planar cell polarity system. Although the molecular
details of these systems are distinct, these differences actually served
to emphasize some of the conceptual similarities.
The PIN proteins, which made an appearance in all but one of the
plant polarity talks, were introduced by Ottoline Leyser (Sainsbury
Laboratory, Cambridge, UK) in connection with the grand
coordinator of plant development and long-range tissue polarity,
auxin. As plasma membrane auxin efflux facilitators, PIN proteins
promote the movement of auxin out of cells. Their localization on
one face of the cell is both a cause and consequence of auxin
distribution and of tissue polarity (Fig. 1E). During the initiation of
leaves in Arabidopsis, PIN1 protein accumulates in a polar fashion,
towards the direction of highest auxin concentration (thus moving
auxin against a gradient); in vein formation, however, PIN1 orients
in the direction of highest flux (Bennett et al., 2014). How a single
protein reads auxin information in these different ways is unclear,
but turning to other plants in which these activities are under the
control of different isoforms might clarify the structural and
regulatory issues (O’Connor et al., 2014). From a more conceptual
standpoint, Przemyslaw Prusinkiewicz used the distribution of plant
PIN proteins in the shoot apical meristem during phyllotactic pattern
formation in Arabidopsis and other plants as a motivating example
to propose a mathematical definition of polarity as a combination of
vector and tensor quantities.
On the majority of the adult Drosophila cuticle, cellular
projections called trichomes show a remarkably well coordinated
polarity, generally pointing toward the distal end of extremities
or the posterior of the body. This polarity, which is referred to as
planar cell polarity (PCP), is evident in other Drosophila
structures, and conserved mechanisms function to regulate
polarity in other organisms including vertebrates. Two molecular
modules in Drosophila contribute to PCP signaling: the ‘core’ PCP
module and the Fat (Ft)/Dachsous (Ds)/Four-jointed (Fj) modules
(Fig. 1C,D). Each displays molecular polarity at the level of
individual cells. In core PCP signaling, complexes of Frizzled (Fz)
on one cell and Van Gogh (Vang; also known as Strabismus) on the
neighbor, plus associated cytosolic factors, are bridged by the
atypical cadherin Flamingo (Fmi; also known as Starry night).
These complexes segregate into a highly asymmetric distribution
with Fz predominantly on one side of the cell and Vang on the other
(Strutt and Strutt, 2009). Ft and Ds are atypical cadherins that form
heterodimeric bridges that couple cells to each other, and, much
like the core PCP system, these proteins also achieve subcellular
asymmetry (Matis and Axelrod, 2013). Intensive study has led
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ABSTRACT
MEETING REVIEW
Fig. 1. Generalized model for generating coordinated cell polarity.
(A) Polarity within a single cell can result from recruitment to the correct
region of the membrane (or the localized activation) of two polarity proteins,
A and B (where the asterisk indicates an activated and membrane-associated
form and dashed arrows the cycling between forms), together with positive
feedback, for example by cooperative recruitment, and with mutual inhibition.
(B) If A* and B* interact across cell junctions (as indicated by linkers between
cells), then this system can also couple polarity between neighboring cells.
(C) Evidence exists for positive and negative feedback as well as for coupling
among the core PCP proteins; for simplicity, only Fz and Vang are shown.
Fz on one cell and Vang on the other are bridged by Fmi. (D) Within the Ft/Ds
polarity system, weak polarization results from graded expression of Ds and Fj
(not shown). Coupling between Ft and Ds in adjacent cells has been
demonstrated, and evidence for intracellular mutual inhibition mediated by
Bulge (Bul) was presented at the workshop. Positive feedback is inferred from
observed clustering, but has not otherwise been demonstrated. These
activities could amplify polarization. (E) PIN1 accumulates in membranes
adjacent to cells with higher auxin concentrations, pumping additional auxin in
that direction. Because of the thick cell wall (gray box) between cells, coupling
between cells cannot be direct, although auxin itself has been modeled as the
mobile mediator of plant cell coupling. (F) ROP2 and ROP6 show reciprocal
recruitment to convex and concave regions, respectively, of opposing cell
membranes in pavement cells. They mutually inhibit each other’s activity
indirectly via activities of their downstream cytoplasmic effectors, and their
opposite effects on local expansion lead to interdigitation. The mechanism of
coupling has been modeled as differential sensitivity to auxin, as well as a
positive feedback through recruitment of PIN1 to ROP2 regions.
Development (2014) 141, 3298-3302 doi:10.1242/dev.111484
the origins of eukaryotic cells. Buzz Baum (UCL, London, UK),
working in collaboration with David Baum (University of
Wisconsin, Madison, USA), suggested a dramatic rethinking of
the mechanism by which archaea and eubacteria combined to form
eukaryotes. Rather than the engulfment of eubacteria by archaea, as
currently envisioned (Williams et al., 2013), it was suggested that
archaea might have formed multiple cytoplasmic protrusions
that surrounded symbiotic eubacteria in what was initially an
extracellular compartment. In the model, the protrusions would have
become the cytoplasm, with the invention of nuclear pore
complexes separating them from the archaeal cell body, now the
nucleus. Membrane fusion between cytoplasmic protrusions could
then have brought the eubacteria into the cell interior, converting the
nearby extracellular space into endoplasmic reticulum. Such an
origin of eukaryotes would have necessarily resulted in a highly
compartmentalized cytoplasm that could have been polarized by the
selective distribution of mRNAs into individual compartments.
Stan Marée [John Innes Centre (JIC), Norwich, UK] discussed a
well-studied example of individual polarized cells. Fish keratinocytes
will migrate when plated in culture, forming distinct front and back
with different cytoskeletal dynamics, and changing direction when
they encounter obstacles. When keratinocytes are enucleated, they
can rest in an isotropic state, but when physically prodded they
polarize and initiate migration that can last for hours. Marée
presented a model involving feedback loops between small
GTPases in active and inactive forms that will produce
spontaneous polarization (Walther et al., 2012). The introduction
of communication by diffusion at appropriate scales makes the
model stable in the contexts of cell migration and upon collisions
that change the shape of the cell. Simulations show that the model
requires only small signals to change direction, but polarization is
very hard to break once initiated (Marée et al., 2012). While this
model stably predicts two compartments, i.e. front and back,
Veronica Grieneisen (JIC, Norwich, UK) argued that a similar
mechanism operating within the confines of a highly irregularly
shaped cell, such as an Arabidopsis pavement cell, can result in
multiple front domains in protruding parts of the cell (Fig. 1F).
These two examples showed high convergence in the behavior of
plant and animal polarity systems at both the conceptual and
physical levels, as both invoked the switch-like properties of small
GTPases. Dominique Bergmann (Stanford University and HHMI,
Stanford, USA) introduced a polarity-generating system in the
Arabidopsis epidermis that relied wholly on proteins unique to
plants; strikingly, however, the dynamics of these novel molecules
in polarity generation could be modeled in terms of conserved
positive-feedback loops or other simple motifs analogous to those
used in animal or yeast cell polarity.
Intrinsic polarity and polarity in single cells
After opening discussions that laid out the key questions, sessions
were organized roughly along a gradient of developmental
complexity, with auxin and PCP components making appearances
throughout. A recurring discussion topic was the question of to what
extent cell polarization is intrinsic in various systems. Several clear
examples of single-cell polarization were discussed, beginning with
a provocative hypothesis talk suggesting the possibility that cell
polarization in eukaryotes might have been a natural consequence of
As cells come into contact, they are often seen to coordinate their
polarities. Two examples of coordinated migration were discussed.
Tadashi Uemura (Kyoto University, Japan) described the Dsdependent coordinated posterior migration of Drosophila larval
epidermal cells. Although its role here is not yet known, Ds in other
contexts was discussed at length, as described below. John Robert
Davis (Stramer laboratory, King’s College London, UK) described
an ‘intercellular clutch’ that coordinates contact inhibition of
locomotion between hemocytes in the Drosophila embryo (Davis
et al., 2012). As two hemocytes migrate toward and contact each
other, an adhesive junction is formed between them. These
junctions couple to treadmilling actin, applying tension and
inducing stress fibers as polarity is reversed. Both cells must be
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From single cells to tissues
to substantial advances in understanding how they generate
coordinated polarity, but much remains to be understood about
their function.
initially migrating toward each other or neither will respond. The
signal that reverses polarity is not yet known.
Katie Abley (Coen laboratory, JIC, Norwich, UK) laid the
groundwork for considering models of Drosophila PCP and
plant auxin-mediated coordinated polarization. Inspired by and
building on a mechanism of cell polarization with intercellular
coupling originally proposed by Hans Meinhardt, she presented
computational models showing that intracellular partitioning
combined with cell-cell coupling, and with overlaid gradients or
boundary conditions, can reproduce coordinated cell polarization
reminiscent of that seen in Drosophila PCP or in a variety of auxinmediated events in plants (Abley et al., 2013). Various modes of
cell-cell coupling, sharing a similar conceptual basis, can be
envisioned and, depending on how much of the intracellular
partitioning mechanism is preserved, uncoupled cells may or may
not retain the ability to partition (Fig. 1).
Polarity in tissues – intercellular polarity coupling and
feedback
Intercellular coupling mechanisms were then extensively discussed
in the contexts of Drosophila PCP and plant auxin signaling. Laura
Brown (Jönsson laboratory, University of Cambridge, UK) showed
that a model of auxin signaling in which the auxin efflux channel
PIN on one cell surface repels PIN on the neighboring cell, together
with the auxin-dependent regulation of PIN production, can
produce realistic leaf venation patterns given an auxin source and
sink. Przemyslaw Prusinkiewicz built a similar coupling model in
which high auxin produces elevated PIN in the neighboring cell,
inducing efflux and reinforcing the high auxin level. His model,
together with growth, could replicate the beautiful spiral pattern of
the shoot apical meristem. The molecular foundations of this
coupling remain to be determined. Similar cell-cell coupling
principles are evident in both the core and Ft/Ds/Fj systems in
PCP signaling, where Fz and Vang complexes assemble on
opposing membranes and oppositely oriented complexes show
mutual inhibition that depends on a set of cytosolic core factors
(Strutt and Strutt, 2009) (Fig. 1).
Nick Monk (University of Sheffield, UK) addressed whether the
simplest forms of these coupling and feedback mechanisms, predicted
to show bistability at intercellular junctions, can produce realistic tiling
patterns in 2D cell arrays. In biological systems, junctions
perpendicular to the direction of polarization are often seen to each
accumulate intermediate levels of signaling components, a result not
simply reconciled with bistability. Monk suggested that networks
including both positive and negative feedback could produce
tristability that is compatible with this outcome (Jaeger and Monk,
2014). This engendered discussion of a variety of underlying
assumptions that could lead to additional investigation. Nonetheless,
Jeff Axelrod (Stanford University, Stanford, USA) presented
evidence that oppositely oriented core PCP complexes directly
interact through both positive and negative feedback, with negative
feedback mediated by endocytosis of the disrupted complex.
Three participants discussed aspects of the Drosophila Ft/Ds/Fj
system, where the current model is that the phosphorylation of both Ft
and Ds by Fj makes Ft a stronger ligand and Ds a weaker ligand
for the other. Therefore, oppositely oriented gradients of Fj and Ds
both promote asymmetric orientations of Ft-Ds heterodimers with
respect to the expression gradients. An open question among
experimentalists in the field is whether this simple model can produce
quantitatively sufficient asymmetry. David Sprinzak (Tel Aviv
University, Israel), using a synthetic approach, reconstituted
vertebrate Ft-Ds interactions in cultured pairs of cells, and the
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Development (2014) 141, 3298-3302 doi:10.1242/dev.111484
measured dynamics suggested cooperativity. Furthermore, he
showed dramatic increases in the stability of bound complexes as
confirmed by fluorescence recovery after photobleaching (FRAP). In
vivo FRAP experiments reported by David Strutt (University of
Sheffield, UK) confirmed the effect of Fj previously measured
in vitro. He then described a mass action model based on the
mechanism outlined above that produces low-level subcellular
asymmetry. Various scenarios for increasing asymmetry were
examined, such as including diffusion and production/degradation
dynamics, or incorporating feedback. Notably, it is challenging to
incorporate feedback in a way that produces a similar response across
the entire field. Mariana Rodrigues Campos (Thompson laboratory,
CRUK-London Research Institute, UK) then described experimental
results suggesting that a novel ubiquitin ligase is recruited by Ft and
acts on Ds and the associated Dachs protein, providing a plausible
feedback amplification mechanism. Substantial attention to this
mechanism is likely to provide a deeper understanding in the
near future.
Two talks discussed how polarity is maintained through
development. Peter Lawrence (University of Cambridge, UK)
described work showing that between the fly larval molts the
denticle polarity patterns are stable, yet the epidermal cells rearrange
substantially. To achieve this, the fates of individual cells and their
polarities change between molts (Saavedra et al., 2014). Danelle
Devenport (Princeton University, Princeton, USA), studying PCP in
mouse skin, identified a cell cycle-dependent kinase pathway that
facilitates maintenance of PCP through cell division by regulating
Celsr1 (the mammalian homolog of Drosophila Fmi) internalization
and redistribution to the membrane.
The role of forces in polarity
The interplay between mechanical forces and molecular signaling
systems in shaping cell polarization was examined by numerous
meeting participants (Fig. 2). In the Drosophila embryo, elongation
of the germ band is accomplished by polarized intercalations of cells.
The polarized accumulation and activation of MyoII selectively at
anteroposterior cell boundaries drives intercalation by promoting
shrinkage of those boundaries until they form four-cell junctions that
resolve irreversibly in the opposite orientation (known as T1
transitions; Fig. 2C). Thomas Lecuit (IDBML, Marseille, France)
described pulsatile apical actin flows operating during germ band
elongation, similar to those reported to propel apical constriction
(Martin et al., 2009) except that the flows are anisotropic, alternating
between anterior and posterior cell boundaries, suggesting a model
for the accumulation and/or activation of MyoII at these locations
(Levayer and Lecuit, 2013). Additional evidence suggests that an
activating signal is required asymmetrically at these junctions. Later
in the Drosophila embryo, as the stripes of Wingless (Wg)expressing and Engrailed (En)-expressing cells (which define the
segments of the animal) are established, mixing between these cell
populations is not seen. Bénédicte Sanson (University of Cambridge,
UK) described actomyosin cables that run on either side of the Wg/En
cell junctions (Monier et al., 2010). These cables are under tension,
forcing the border into a straight line and presumably blocking
mixing. Cell divisions at the Wg/En boundary fail to disrupt this
border. Unlike the earlier case, in which MyoII enrichment causes
junctional rearrangements, these junctions remain stable. The reason
for the difference in junctional stability is not yet known.
Boris Shraiman (Kavli Institute, University of California at Santa
Barbara, USA) presented a theoretical model of the early phase of
germ band extension described by Lecuit. Assuming that junctional
stress is dominated by actomyosin and is in approximate equilibrium,
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MEETING REVIEW
Development (2014) 141, 3298-3302 doi:10.1242/dev.111484
Fig. 2. Interactions between polarity and
mechanics in development. (A) Within the
plant shoot meristem, tissue arrangements
and the turgor pressure intrinsic to each cell
lead to predictable mechanical stress
patterns. As new leaf primordia are formed at
the flanks of the meristem, changes in tissue
architecture (bends, folds, outgrowths)
change the stress directions. Both cortical
microtubules and PIN1 localization become
aligned in response to mechanical cues
within the cells. Here, the overall microtubule
orientation in multiple cells is indicated
(orange arrows). (B) Mechanics and
morphogenesis in individual Arabidopsis
pavement cells. A uniform outward force
drives isotropic cell expansion. Microtubules
guide the deposition of cellulose, which is the
main force-resisting cell wall polymer,
resulting in localized restriction of growth and
the formation of lobes. (C) In animal cells,
cytoskeletal activity both contributes to and
responds to polarity. In the Drosophila germ
band embryo, actin-dependent flows
establish polarity of actomyosin activity.
Contraction of the actomyosin then shrinks
individual cell junctions, or groups of cell
junctions, leading to polarized
rearrangements of cell packing and
contributing to elongation of the embryo.
King’s College London, UK) described a group of cells that
delaminate from the mouse tooth bud and form arches over the
apical surface of the bud. These cells appear to undergo a
convergent extension. Cutting experiments demonstrate that they
are under tension, thereby providing force to drive bud formation.
Tadashi Uemura described studies on the mouse oviduct, which has
invaginated ridges running parallel to the length of the tube.
Mutations in Celsr1 disrupt the organization of these ridges,
suggesting that planar polarity might contribute to ridge formation.
Linked or independent systems
In several systems, the possible or apparent linkage between distinct
signaling systems in controlling polarity was discussed. Pluripotent
mouse epiblast cells differentiate in culture into a variety of cell fates
and this is likely to depend on transcription factor levels,
intercellular signals and cell polarity, as manipulating polarity
alters differentiation. Guillaume Blin (Lowell laboratory, University
of Edinburgh, UK) is investigating whether models integrating data
regarding cell contacts, cell shapes and polarities can predict the
differentiation pathway of individual cells in these cultures.
Claire Grierson (University of Bristol, UK) discussed the
interaction of Rho of plants (ROP) and auxin in determining
the subcellular localization of lateral root hair outgrowth. Root hairs
normally grow laterally from the distal end of cells where
subcellular auxin is expected to be highest. A patch of ROP
accumulates at the site to direct growth. Ectopic auxin and changes
in ROP levels can alter the number and location of outgrowths.
Grierson suggested that a Turing type model can potentially account
for the wild-type pattern, as well as for a number of interventions
that alter root hair localization (Payne and Grierson, 2009). The
model is awaiting more quantitative tests.
The anteroposterior axis of planarians depends on Wnt signaling,
with high-level signaling inducing tail formation. Along the body
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DEVELOPMENT
and including negative tension contributions from E-cadherin
bridges, he showed that mechanical stress feedback onto myosin
and cadherin levels can spontaneously produce an anisotropic state,
characterized by the presence of parallel myosin cables with elevated
tension. This anisotropic state is unstable and progresses toward the
formation of cellular ‘rosettes’ via unresolved T1 processes.
Resolution of these four-cell (and higher) cellular junctions requires
reorganization of cable structures. Further work is needed to relate the
dynamics of the cables to the global structure of cell flow.
The interplay between mechanics and polarity in plant cells is
dominated by the presence of a rigid cell wall that eliminates the
possibility of the morphogenetic movements seen in animals.
Nevertheless, membrane tension in both animal and plant cells can
act as a common cue to channel cell polarity (Asnacios and Hamant,
2012). Mechanistically, this involves the cytoskeleton. As described
by Olivier Hamant (INRA, Lyon, France), animal cells rely on actin
to generate cortical tension and shape, whereas plants employ a
microtubule-based cortical array whose dynamics and impact on
cell wall synthesis allow the cell to efficiently resist mechanical
forces, leading to directional growth and tissue morphogenesis
(Fig. 2A,B) (Hamant et al., 2008; Sampathkumar et al., 2014). Two
specific manifestations of these principles were described in the
emergence of lateral roots by Amaya Vilches Barro (Maizel
laboratory, University of Heidelberg, Germany) and in the
generation of new organs in the shoot by Jan Traas (INRA, Lyon,
France), who also integrated auxin and PIN protein localization,
noting that both microtubules and PIN protein localization
responded to mechanical force, but that the disassembly of highly
aligned microtubule arrays was actually a requirement for symmetry
breaking and the auxin-driven outgrowth of organs (Heisler et al.,
2010; Burian et al., 2013).
The role of polarity in morphogenesis was further discussed in
two vertebrate systems. Eleni Panousopoulou (Green laboratory,
MEETING REVIEW
Ideas, themes and questions
The workshop was structured in a way that was intended to foster
discussion, and indeed discussions were plentiful and invigorating.
One recurring theme was the extent to which the collective polarization
of cells is built upon intrinsic intracellular polarization mechanisms.
Modeling can produce collective polarization either with or without
intrinsic cell polarization. Thinking about this problem will require
consideration of hierarchies of polarizing mechanisms. For example,
an intrinsic polarizing mechanism might operate strictly downstream
of a system that couples the polarities of neighboring cells. However, it
is possible that polarity-coupling mechanisms might also have intrinsic
polarizing activities. An experimental challenge will be to identify
paradigms that will determine whether such cell-intrinsic mechanisms
exist in a given system, and for this Boris Shraiman proposed an
experimental approach. As the various collective polarization systems
are investigated further, answers to this question will begin to come
into focus.
Another common theme was the potential functions and
consequences of clustering of transmembrane signaling structures
into discrete puncta. The formation of puncta might simply be a
consequence of amplification mechanisms that use positive
feedback, but one could envisage other important implications of
these structures. For example, modelers tend to think of any given
cell-cell interface as a homogenous structure with identical
properties throughout, but the subregionalization of interfaces
could challenge this view.
For the field to move forward, a number of technical advances are
required, many focused on the acquisition of quantitative data in
living tissues. For Drosophila PCP, knowing the stoichiometry of
polarity complexes would enable a better evaluation of current
models of activity. New imaging techniques, such as superresolution microscopy, were frequently discussed, and new
applications of fluorescence correlation spectroscopy (FCS) or
iterative raster scanning, as presented by Rosangela Sozzani (North
Carolina State University, Raleigh, USA) for her work following
cell fates in plant stem cell niches, hold promise for determining
in vivo protein complex dynamics. While the drive towards more
definitive assignments of proteins to specific places and genes to
specific networks was a goal of many participants, Eric Siggia
(Rockefeller University, New York, USA) advocated instead for
‘messy’ genetics. Although geneticists tend to prefer fully penetrant
and expressive phenotypes, he suggested that quantitative
information from partially expressive and/or penetrant phenotypes
is a richer source from which to derive model parameters. As an
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example, he pointed to the fly eye, where quantitative data about
ommatidial rotation are available.
In summary, the meeting pointed to a number of commonalities
among the diverse mechanisms of collective cell polarization and,
perhaps more importantly, may catalyze closer dialog between the
various constituencies as they work toward the common goal of a
better understanding of these processes.
Competing interests
The authors declare no competing financial interests.
Funding
Work in the authors’ laboratories is funded by the National Institutes of Health
(NIGMS) (J.D.A. and D.C.B.) and by the HHMI/Gordon and Betty Moore
Foundation (D.C.B.).
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DEVELOPMENT
length, cells also have a PCP that is evident in the orientation of the
ciliary rootlets and appears to depend on conserved core PCP
components. Preliminary evidence suggests that local PCP requires
the Wnt signal. Sarah Mansour (Rink laboratory, MPI-CBG,
Dresden, Germany) discussed the possibility of cut portions of
body to ‘remember’ their orientation and regenerate accordingly by
virtue of their planar polarity. This hypothesis remains to be tested.
Suzanne Eaton (MPI-CBG, Dresden, Germany) has examined the
relationship between the Ft/Ds and core PCP systems in the fly
wing. By mapping their orientations throughout wing development
in wild-type and mutant conditions, she concludes that at different
developmental stages the systems are either coupled or uncoupled
and that, when coupled, one or the other dominates to determine
orientation in a stage-dependent way.
In each of these cases the level of complexity is high and,
although for some, evidence of coupling is provided, the
mechanisms for coupling remain to be determined.
Development (2014) 141, 3298-3302 doi:10.1242/dev.111484